1. Field of the Invention
This invention generally relates to methods and apparatus which employ high energy gas bubbles to generate underwater, sharp, impulsive acoustic signals especially useful in marine seismic exploration. The invention relates more particularly to improve such methods and apparatus wherein secondary acoustic pulses or signals are suppressed.
2. Description of the Prior Art
Certain seismic sources such as explosives, air guns, gas exploders, etc., are purposely fired deep under water. It is well known that such firing creates a gas bubble or cavity and that the water acquires oscillatory energy which generates acoustic pressure wavelets, each consisting of a desired "primary" acoustic pressure pulse having an amplitude P.sub.o, which is especially useful for most seismic exploration work, and which is followed by an oscillating succession of undesired "secondary" (sometimes called "bubble") acoustic pulses of decreasing amplitude. In this specification, the words "bubble" and "cavity" will be used interchangeably.
For example, an air gun explosively releases a high pressure gas bubble having an energy E.sub.a into the water which creates a desired primary pressure pulse having a maximum amplitude P.sub.o. After the released high-pressure gas bubble impulsively contacts the surrounding water, it continues to expand as the water first accelerates outwardly and later decelerates until the cavity attains a maximum volume and comes to rest, at which time the pressure within the cavity is much less than the surrounding hydrostatic pressure. At this point, the cavity has attained a maximum volume V.sub.m.
When the expanding bubble reaches its maximum volume, there is practically a vacuum inside the bubble, the kinetic energy of the surrounding water is zero, and this water possesses maximum oscillatory potential energy which, if not suppressed, will change into kinetic energy, back into potential energy, etc., for a duration of several cycles, each having an oscillatory time period T.
The water gains maximum potential energy at 1/2 T, at which time the water is ready to change course and rush inwardly to implode the gas in the bubble. After one complete cycle, i.e., at time T, the bubble is recompressed into a relatively small-diameter, high-pressure bubble. The surrounding water comes to an abrupt stop resulting in a first positive acoustic secondary pulse having an amplitude P.sub.2 which is mainly dependent upon the maximum kinetic energy acquired by the inwardly-moving water. The less kinetic energy acquired by the water, the smaller the amplitude P.sub.2 will be.
Thus, the secondary pulse phenomenon occurs when the surrounding water first violently implodes the bubble to a minimum diameter or volume, the kinetic energy is again zero and the potential energy is mainly contained within the recompressed gas inside the bubble. This potential energy causes the bubble to again explode in its oscillatory scheme as previously described.
In this manner, the oscillatory energy stored in the water produces several successive secondary pulses of decreasing amplitude until a portion of the energy of oscillation becomes dissipated by natural processes, such as turbulence, and the remaining portion is consumed to produce the undesired secondary pulses.
The number of such bubble explosions (expansions) and implosions (contractions) may vary, but typically four to six significant secondary pulses can be expected after each primary pulse P.sub.o which is generated by the seismic source. Hence, a substantial portion of the acoustic energy released by the seismic source goes to waste because only a portion of the energy contained in the released gas is used to produce the desired primary seismic pulse P.sub.o, while the remaining and substantial portion of the energy becomes converted into harmful secondary seismic pulses having amplitudes P.sub.2 which must be suppressed.
In seismic exploration, both the primary and secondary acoustic pulses act as distinct acoustic disturbances which travel in the water in all directions, penetrate the earth, strike one or more rock formations or reflectors, and then return into the body of water. The primary and secondary pulses produce reflected seismic wavelets. But, since the secondary pulses and their reflected waves occur at times when the reflected primary waves also return from the subterranean reflectors, the secondary pulses and their reflected wavelets interfere with the reflected primary waves. Further, because the reflected secondary waves and the reflected primary waves are similar in shape, no practical way has yet been found for distinguishing between them.
In conducting marine seismic surveys, the reflected primary and secondary seismic waves are sensed by detectors within a towed streamer cable. The detectors faithfully transform the received acoustic seismic waves into corresponding electric signals which are processed into seismic traces that contain appreciable noise. This noise is due mostly to the oscillatory secondary pulses which accompany each primary pulse. Under these noisy conditions, computations of the depths at which the rock formations lie become very difficult and sometimes altogether impossible. This noise hinders the main object of the seismic exploration, which is, of course, to identify the various subterranean formations from an interpretation of the seismogram sections produced by the seismic survey.
The secondary-to-primary ratio amplitude P.sub.2 /P.sub.o is the benchmark by which all marine seismic sources are measured as to bubble suppression. An "ideal" source is said to be that source which has a ratio P.sub.2 /P.sub.o =0 for a frequency range from 0-125 Hz. Therefore, the extent to which a particular seismic source approaches the ideal seismic source can be readily measured by measuring its P.sub.2 /P.sub.o ratio.
An ideal seismic source produces a single, short, sharp acoustic impulse having sufficient energy and no secondary pulses. Sharp impulses are needed to improve the definition of seismic reflections, because resolution is inversely proportional to the time-width of the impulse: the larger the time-width of the impulse, the less desirable it is. Fired near the water surface, a dynamite charge or other similar concentrated explosion closely approximates the ideal seismic source, because the bubbles resulting from each explosion are vented immediately into the atmosphere, hence there are no bubble implosions. If not fired near the water surface, explosive seismic sources will produce undesired secondary pulses, unless some form of implosion suppression is utilized. Explosive seismic sources include explosives, air guns, gasguns, expandable sleeve devices in which propane and oxygen are mixed to cause internal combustion, etc. All of these share the common bubble problem for which there has been no fully satisfactory solution, even though there has been a long-felt need to find a mechanism to enhance the desired primary pulse at the expense of the undesired secondary pulses.
In the absence of such a mechanism, many attempts have been made in the past twenty-five years or more by the energy industry and their seismic contractors to develop techniques for reducing the burden, financial and technological, imposed by the generation of the undesirable secondary pulses. These efforts have been directed toward attenuating the oscillatory secondary pulses and/or to reduce their ill effects. From the initial introduction of marine seismic sources, there has been a continuous need for effective and economical bubble suppression devices. That need and the various solutions offered to fill that need are well described in the technical and patent literature.
One early mechanical technique attempt to prevent the secondary pulses from traveling vertically downward towards the water bottom involved a method whereby the gas bubble source was substantially enveloped in a container or cage having perforations, such that the expanding gas bubble would have to do work in order to force water through the perforations. The work performed by the expanding gas bubble dissipated its internal energy, so that the ensuing secondary pulses would have reduced amplitudes. This technique has been used in a seismic source trademarked FLEXOTIR. A serious limitation inherent in this technique has been that the desired primary pulses also become reduced in strength because they can travel freely only through the available perforations. Also, the perforated cage becomes subjected to rapid deterioration, due to the great stresses to which it becomes subjected when large differential pressures become exerted across its wall.
Various software programs have also been developed, for example, in connection with the MAXIPULSE (trademark) seismic source, which utilize fast and powerful digital computer which produce seismograms from which the detected noisy seismic waves, caused by the deleterious bubble effects, have been removed so that the seismograms can be easier interpreted by the geophysicists. However, running such programs requires the use of expensive computer time and manpower, see e.g., U.S. Pat. No. 3,592,286.
Other prior art techniques have been based on air being injected into the expanding bubble for shaping the secondary pulses. The known applications of the air injection technique have led prior art workers to very disappointing results and most of them were abandoned.
Due to the inefficiency or impracticability of known bubble suppression techniques, the seismic industry has also been obliged to employ a "tuned" array of seismic sources. Typically, these sources are air guns of markedly different sizes. In theory when all such seismic sources are situated in a tuned array, and then fired simultaneously, the amplitude of the resulting primary pulse of the array will be equal to the sum of the amplitudes of the individual primary pulses generated by the individual acoustic sources. Conversely, the amplitudes of the secondary pulses will theoretically be reduced because (1) they are not in phase, (2) they occur at different times, and (3) they have random frequencies.
Though the aforedescribed array technique has been the standard in this art, this technique has presented serious drawbacks since the resultant seismic signature is only a composite of individual sources, each source lacking a narrow, sharp acoustic pulse as required. Also it has been very expensive to build such an array because it has required a large number of differently-sized air guns, as well as heavy and expensive air compressors, to provide the appropriate volume of pressurized gas consumed by the large number of airguns. Additionally, there is also a need to maintain on boat a large inventory of spare parts to keep the differently-sized sources operational. The spare part problem is very serious, because in many parts of the world they are not available and they must be flown in from the home base. Many parts break down daily and some weekly due to salt water, pollution, unsuspected debris, high pressure, etc.